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PageRank is just one of the factors used to determine positions in the SERPS, otherwise Google, Yahoo et al would outrank every other site on the web for every search term.
My guess is those lower PR pages that are outranking yours have either more inbound links than yours or links from higher PR pages than yours and/or more links with the keywords in the anchor text.
And of course their on-page SEO could be better than yours. In short, PR is more of a tiebreaker than anything else.
Look at it this way: Apple Computer's home page has a PR10, but you wouldn't expect it to turn up in a search for "fresh oranges", would you?